You could use something like an arduino - a standalone small single chip computer with lots of I/O pins and a usb - and write something yourself
or you could buy a USB data acquisition board from someone like National Instruments

The arduino itself is programmed in 'sort of' C - with it's own free development tools. A program to just receive a word on the serial port and turn on the appropriate pins would only be a dozen lines.
Then you c# program on the pc just has to write words to the USB, normally you would install a driver to make the USB look like a serial port.
If this is for real work rather than just a project it's probably easier to just buy a USB daq board with a driver.